Centrifugation is well known as a technique for separation of the constituents of a liquid suspension where the constituents have slight differences in density. Centrifugation systems are widely used in biomedical applications, one of the most important of which pertains to fractionation of the constituents of blood, which is a delicate and complex substance carrying suspended cellular and other matter of relatively small density differences. When blood samples are taken, they are often separated under high centrifugal forces by spinning at extremely high rotational velocities, such as 5,000 r.p.m. and high separation forces, such as 5,000 g, for 5-10 minutes. This produces, in the sample, layering of heaviest density red and white cell matter relative to the lighest density constituent, plasma, with a thin layer (sometimes called the "buffy coat") of platelets and white blood cells between. Specific cell types can be removed from a centrifuged bag by expressing the separated zones into individual containers.
While centrifugation is commonly carried out as a batch process, there are many continuous centrifugation systems in general use, although those suitable for blood handling are specially adapted for that purpose. When it is desired to extract an intermediate density constituent in a continuous process, a probe or knife edge at the appropriate position can be used for separation of a selected layer in a continuous centrifugation machine. Such systems are complex, particularly where the intermediate density layer is present only in a low proportion. Chromatographic techniques, which separate constituents successively with time, are also known but are again complex.
For extracting platelets from whole blood, an improved system has recently been devised that is the subject of a patent application entitled "CLOSED HEMAPHERESIS SYSTEM AND METHOD", U.S. Ser. No. 644,032, filed Aug. 24, 1984, by Donald W. Schoendorfer et al. In accordance with this system and method, platelet rich plasma is separated from blood by a first step in which blood is fed into a biologically closed structure having an interior double-walled rotor within a concentric housing. The preferential flow path is between the walls of the rotor, as opposed to the path between the outside of the rotor and the housing, so that centrifugal layering and separation of platelet rich plasma are established within the rotor. The platelet rich plasma may then be filtered in a rotary membrane system to the desired final platelet concentration, deriving plasma as an added product. The size, efficiency and simplicity of this system enable the operative parts to be fabricated as low cost disposables. The system is also operable in real time during a donation procedure to extract the platelet concentrate output while returning the remaining constituents of the blood to a patient or donor. The work leading to the present invention was undertaken to obtain important gains in platelet concentrate levels, efficiency and throughput.
Obtaining blood platelet concentrations represents a particularly critical example of the problem of extracting an intermediate density substance from both lighter and denser matter in a liquid suspension. Blood platelets are used for analytical, therapeutic and other purposes. In modern applications it is highly desirable to reinfuse platelet-depleted blood into a donor in a procedure using disposable separators and taking a minimum amount of time. Automated or semi-automated plateletpheresis systems, such as the Model V-50 of Haemonetics Corporation, the I.B.M. 2997 marketed by Cobe Laboratories, and the CS3000 marketed by Fenwal Laboratories operate by these means. These systems, however, are expensive and complicated to run. Because plasma has a density of 1.0269 and platelets have a density of 1.03 (red blood cells have a density of 1.10), the difficulty of fractionation has heretofore precluded the usage of substantially simpler and less costly systems. Thus manual plateletpheresis, which has been in use for more than 25 years, is still employed. Here a batch type two-step centrifugation process operating on single units of blood is used with a first lower velocity spin to derive platelet rich plasma, and then with a second higher velocity spin to concentrate the platelets. This not only requires much manual handling, but supplies from separate donors must be mixed in order to obtain an adequate amount of platelets for platelet transfusion.
Human blood normally is composed of about 50% plasma and much less than 1% of platelets in a concentration of approximately 250,000 platelets per microliter of whole blood. Thus when plasma is separated from the blood together with substantially all platelets there will be 500,000 to 550,000 platelets per microliter of plasma (the "Norm"). Platelet concentrate is usually regarded as having approximately 1.1 million platelets, or more, per microliter of plasma. Obtaining plasma that is platelet rich above the norm as well as free of hemolysis, and doing so on a continuous basis compatible with donor flow rates (typically about 50 ml/min) is therefore a most worthy objective. Red blood cells and some plasma can concurrently be returned to the donor as the platelet rich plasma is stored or otherwise made available for platelet transfusion or other purposes. If a high platelet concentrate (e.g. 4,000,000 platelets/microliter) is desired on a real time basis then an in-line rotary membrane filter can be employed as described in the above-mentioned Schoendorfer et al application.
A one-step procedure for extracting platelet rich plasma from whole blood solves a very difficult problem, and the procedure should moreover be amenable to usage with other applications where it is desired to selectively extract one constituent or target material from both heavier and lighter matter in a suspension.